Indications of Endoscopic Submucosal Dissection for Undifferentiated Early Gastric Cancer: Current Status and Future Perspectives for Further Expansion

<b><i>Background:</i></b> Endoscopic submucosal dissection (ESD) is a widely accepted and minimally invasive treatment for early gastric cancer (EGC) without the risk of lymph node metastasis (LNM). However, undifferentiated-type EGC (UD-EGC) is considered to have a relatively high risk of LNM. Recently, the Japan Clinical Oncology Group conducted a nonrandomized confirmatory trial (JCOG1009/1010) to evaluate the efficacy and safety of ESD for UD-EGC. Herein, we review the results of JCOG1009/1010 and the possibility of further expanding the indications for ESD. <b><i>Summary:</i></b> JCOG1009/1010 showed excellent technical results and 5-year overall survival in patients with UD-EGC. Based on the results, ESD for UD-EGC (cT1a) of ≤2 cm without ulceration was technically feasible and acceptable for standard treatment instead of gastrectomy with lymph node dissection. A review of the EGC of mixed histological type (mixed EGC) suggested that the mixed EGC might have worse biological behavior than the pure histological type. In cases of intramucosal EGC with pure signet-ring cell carcinoma or presenting a double-layer structure, the risk of LNM might be relatively low. Thus, there is a possibility of further expanding the indications or curative evaluations. In the case of UD-EGC after noncurative resection, the data suggest that the eCura system may be applicable to UD-EGC; however, due to the small number of cases, further study is warranted. <b><i>Key Message:</i></b> This review summarizes the present knowledge regarding indications for UD-EGC and the possibility of further expanding them.